In relation to a typical monolithic structure that is provided with a monolithic base that includes a plurality of filtration cells and a separation membrane that is formed on an inner surface of the filtration cells, a technique has been proposed to inhibit adverse effects on strength that result from high-temperature alkali processing by suitable provision of a thickness to the separation layer and a partition wall thickness between two filtration cells (reference is made to PCT Laid Open Application 2012/128218).